Update Dev section - What we’re aiming for

Part of &959 (closed)

Why is this change being made?

@ebrinkman, @kathleentam, and I met to review the Dev Section PIs and the following changes were made to the What We're Aiming For Section:

• We clarified Section XMAUs are Section TMAUs
• We will not be aiming for Paid XMAU SaaS segmentation for dev.manage or dev.manage.analytics as these PIs currently use Redis. Redis data is currently not brought into the data warehouse and therefore we are unable to segment Redis by free/paid.

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per [say-why-not-just-what][transparency]
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